Magnetic tapes are used for data storage in computer systems requiring data removability, low-cost data storage, high data-rate capability, high volumetric efficiency and reusability. Magnetic tape devices include open reels, tape cartridges and cassettes. In the past the container most commonly used to house magnetic tape was the open reel. A simple reel, consisting of a hub upon which the tape is wound and flanges which protect the tape edges, has been used for more than three decades. However, the need for additional tape protection and a reduction in the need for human intervention has led to the use of tape cartridges and cassettes. A cartridge denotes a single reel of tape in a machine-usable container, while a cassette includes a take-up reel as well as the supply reel. Take-up reels are used in magnetic tape drives to spool tape from a tape cartridge or from a reel while information is either written on the tape or read from it.
The need to record large amounts of information to achieve high data rate capability on tapes has been achieved through the use of parallel tracks on a tape. Greater the number of tracks, more information may be stored on the tape. Consequently, track separation on tapes have continued to decrease in order to accommodate more tracks. As a result, it has become very important to control lateral tape motion as the tape passes over a read/write head during read/write operations in order to ensure that the desired track is accurately positioned on the head.
As the tape moves between the supply and take-up reels during operation, it must be guided over the read/write head in a precise manner. Excessive lateral tape motion is undesirable. The transverse position of the tape relative to the read/write head is very important in order to avoid misalignment between the recorded track positions and the head, as such tracking errors may reduce data reliability. During a write operation any lateral motion of tape prevents a straight track from being written on the tape. Further, during a read operation lateral tape motion keeps the read head from being aligned at the center of the desired track on the tape, thus causing data errors.
One approach to solving the problem of lateral tape motion has been to ensure that tape stacks up uniformly on both supply and take-up reels. As shown in FIG. 1 (prior art), it can be seen that as each individual loop stacks, the tape may slide laterally up or down as it spools around the reel, thereby stacking non-uniformly. Non-uniform stacking of tape on the reels causes the tape to experience lateral motion as it passes over the read/write head. When tape spools on a reel, it has to squeeze out the layer of air that is trapped between the outer surface of the tape in the reel and the in-coming tape. Efficient removal of air is important to ensure that tape stacks evenly in the reel. By increasing tape tension and surface roughness, uniformity of stacking may be achieved.
However, the use of higher tape tension to reduce the sliding up and down of the tape as it spools on the reel is not very effective when using thin tapes, as is the norm today. High tape tension increases the head and tape wear.
High surface roughness causes air to be squeezed out easily between successive layers of tape as it spools on the take-up reel. However, increased surface roughness deteriorates data integrity. Therefore, the use of tape having increased surface roughness to control the uniformity of stacking is undesirable.
Another prior art method to control the non-uniform stacking of tape involves making the flange separation of a reel approximately the same as that of the tape width. This approach ensures uniformity of tape stacking by eliminating any redundant space between the flanges that may allow lateral motion of tape as it spools. However this approach leads to excessive tape edge damage due to constant rubbing with the flanges.